The
conclusions (from correlation diagrams) about pericyclic reactions can be
generalized to a set of selection rules for pericylic reactions -

the Woodward-Hoffmann Rules:

These
rules can be expressed in a number of ways.†
A summary for pericyclic cycloaddition reactions is:

p + q

Thermally
allowed

Photochemically
allowed

4n

ps + qa or pa + qs

ps + qs or pa + qa

4n +2

ps +qs or pa + qa

ps + qa or pa + qs

You donít need to learn these for this course!

∑p and q are the number of electrons in the two
p systems which are undergoing the cycloaddition
reaction.†

∑s indicates suprafacial attack with respect to one of the p components, and

∑a indicates antarafacial with respect to one of the p components.†

From the table outlining the rules, when p and q add up to give a number
which can be written as (4n +2), with
n as an integer (0, 1, 2Ö), the
thermal cycloaddition reaction is allowed when it is suprafacial
with respect to both components (or antarafacial with respect to both
components).†